914-918 N Bewley St - REV 1 - Plan

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Illumination Foundation Santa Ana, California Hydrology Report by TAIT & Associates Section 2 Hydrology Design Criteria, Methodology, and Analysis 2 .1 Hydrology Design Criteria and Methodology The existing and proposed condition hydrology calculations were prepared in conformance with the Orange County Hydrology Manual (OCHM), dated 1986 and its Addendum No. 1, dated 1996.The hydrology analysis and small area hydrograph were prepared using the Advanced Engineer Software (AES), a computer software approved by the Orange County Flood Control District that follows the hydrology methodology and criteria from the OCHM. For this analysis drainage area delineations were prepared following existing topog raphy and proposed contours for each analysis, flow paths, areas, and elevation were determined, as well as hydrology characteristics such as soil group and land use and pervious area. The rational method for the existing and proposed conditions for the 25-year and 100-year storm events were prepared for the proposed project. The RATSCX module was used to analyze and route runoff through each subarea using elevations, relative slopes, flow lengths, soil types, and acreage inputs to calculate time of concentration and peak flow rates. The input criteria for the model were as follows: Design Storm: 25- and 100-year storm events Antecedent Moisture Condition (AMC): II for 25-year and III for 100-year SCS Soil Group: A Existing Land Use: 11+ Dwellings/Acre (80% impervious) Proposed Land Use: 11+ Dwellings/Acre (80% impervious) 2 .1.1 Soil Conditions Per the USDA Natural Resources Conservation Service Soil Survey (USDA NRCS), published in 2006, the project site is located within the hydrology soil group of A. Group A Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. The project site location is shown on the USDA Soil Resource map, included in Appendix A. 2.1.2 Special Flood Hazard Zones Per FEMA’s Flood Insurance Rate Map (FIRM) the subject property is located within Zone A which designates areas without an established 1% chance storm flood elevation. Projects located within Flood Zone A areas require the first floor to be elevated a mini mum 2 feet above the natural highest adjacent ground in compliance with FEMA/SBC regulations. Per section 3.2.1 in the Orange County Local Drainage Manual for buildings located within areas that do not list a Base Flood Elevation then the elevation of the lowest floor of the building, including basements, or cellars must be at least 1 foot above the established BFE. If there isn’t a BFE, the 914-918 N Bewley St5/8/2025